There are, in general two main techniques used to extract date juice from fresh dates. One of them is an old traditional technique and the other is relatively new hot extraction technique. The traditional method of extraction is a local method used since ancient times in the Middle East for extraction of date-juice, locally known as ‘Dibs’. In this technique, dates are packed in plastic containers and piled in parallel rows on wood strips in a tightly closed room, which is never opened during the extraction period that extends over two weeks or more. Pressure exerted by date piles, together with the warm room condition, cause extraction of thick date-juice which is collected through a small opening, at the bottom of one side of the room. The yield never exceeds about 10%, and it is used mainly for local consumption. This technique, suffers from many drawbacks such as long extraction time, extending over two weeks or even a month, sanitary conditions are not preserved, as fresh dates and the extracted juice could be subjected to unavoidable dust and insects that might migrate to the closed room, the produced juice is dark and turbid, and the yield is very low (less than 10%).
In the new hot extraction technique a predetermined amount of date is mixed with a certain amount of water (approximately 2.5 water to 1.0 dates), and cooked, in pressure cookers or open pans, then press filtered by a hydraulic press filter, to separate liquid juice from remaining solids. Thin juice thus obtained, is concentrated by evaporating excess water, in open pans. Yield can reach over 70% in this technique, and large quantities of juice can be produced. However, there are some drawbacks to this technique that can be summarized as follows:
a) Use of traditional energy sources, such as gas or coal, in which heat is transferred through the food by conduction. In this case, a substantial amount of heat energy is expended in heating up the pans and the surrounding environment.
b) Using open pans and transferring the boiling mixture from these pans to the press filter could cause hazardous conditions for working personnel
c) No means for controlling and efficient use of the available energy
d) Long extraction time, due to factor (a) above, in which the rate of heating depends on the thermal conductivity of the date-water mixture.
In an effort to speed-up date-syrup extraction process by both the traditional and hot extraction methods, the inventor of the present invention has used microwave heating instead of traditional heating by conduction, for extraction of date juice from fresh dates, using a domestic microwave oven for both extraction and concentration. The results show that using microwaves gave a product superior to those produced by traditional heating in terms of colour and purity of the final product and there is a speeding-up of both extraction and concentration process.
FIG. 1 shows an initial embodiment of a microwave press apparatus already part of the prior art. This apparatus consists of a locally assembled microwave power source (G) comprised of a high power source (S), a magnetron (M), a rectangular waveguide (W) with tunable mechanism for aperture coupling of microwave energy to a microwave-transparent container (C) of fresh dates, seated inside a resonator cavity with adjustable height (R), for press extraction of date-juice, while fresh dates are exposed to microwave heating. The cavity (R) is fed at its bottom end with microwaves guided through an aperture drilled at the center of a detachable lower plate (D).
Although this extraction apparatus gave far superior performance compared to the traditional method for production of date juice from fresh dates, it had a number of disadvantages and limitations, which can be summarized in the following:
i. Microwave power is fed through an aperture of a detachable plate (D), at the bottom of the resonator (R), which makes it difficult in assembling and de-assembling of the apparatus. In addition, any accidental leakage of the extracted juice through the lower bottom end of the resonator could readily be directed through the feeding waveguide thus disrupting microwave passage and cause partial reflection of microwave energy.
ii. The only way of insertion and removal of the fresh date container (C) is through the bottom end of the apparatus, after removing the microwave power feeding assembly and the detachable bottom plate, which is slow, cumbersome and awkward method.
iii. The pressure application mechanism is manually operated for both application and release of pressure (by turning a threaded circular plate clockwise and anti-clock-wise respectively).
iv. The extracted juice remains in the lower conical part of the container (C), thus being exposed to further microwave heating before being drained out. This is not desirable as the extracted juice is already concentrated and does not need any further heating.
v. The support system for the apparatus, composed of four rods and two detachable square plates, had to be de-assembled after each extraction cycle and re-assembled again to start a new extraction cycle.